Its within a new printing of the Disney Villainous: Introduction to Evil set that youll find the foil Promo King John card, which is appropriate seeing as one of the games villains is King John himself
You don't sacrifice that card to get an artifact back
Note that there are no "exile" or "sacrifice" triggers post attack or EOT, they'll stick around until this guy leaves
A variety of creature-toolbox decks rely on Chord of Calling as an instant-speed tutor to find silver bullet answers like Reclamation Sage and Collector Ouphe to hamper their opponents, or to assemble value engines with cards like Yawgmoth, Thran Physician
Likewise, if a creature with backup is given additional abilities, such as by an Aura or Enchantment, they won't be shared with the creature it backs up
You could get Chaos Orb or Lingering Souls, Tolarian Academy or Skullclamp